Cologne finally grabbed their first home win of the season as Milivoje Novakovic's first-half goal was enough to down Mainz.
The home captain returned to action from injury and scored his first Bundesliga goal of the season in the 42nd minute to set up the win.
The victory leaves Cologne, who had not scored in their last two Bundesliga games, in 14th place on eight points.
Mainz, who had won four of their previous six games, are six points better off in seve! nth.
Following a scrappy opening littered with free-kicks, Novakovic had Cologne's first chance after 12 minutes. The Slovenian striker was fed by Fabrice Ehret but Mainz keeper Heinz Mueller was equal to his effort.
Mainz still had troubles creating chances and Cologne grabbed the lead three minutes before the break. Lukas Podolski sent Ehret down the right side of the area and he crossed low into the box, where Novakovic fired first-time past Mueller from six yards.
Novakovic nearly doubled the lead two minutes later but saw a header from a right-wing cross saved by Mueller.
Mainz had a chance 11 minutes after the break but Elkin Soto's 30-yard effort after a free-kick went just over the bar.
Aristide Bance turned and fired narrowly wide right just after the hour mark, and the Burkina Faso native had the visitors' best chance of the game a minute into injury time when his header slipped just wide.
